PHYSICS: C. BARUS 
413 
through the auxiHary clear plate P (to be used preHminarily for parallel- 
izing the mirrors of the system in a way presently to be shown), the 
beam is reflected at a and h by the half silver plates Hi and Hi respec- 
tively to the wide opaque mirror m. The rays now retrace their paths 
or nearly so to be in turn transmitted at a and h by the half silvers Hi 
and Hi. These transmitted pencils similarly impinge on the opaque 
mirror M and the half silver H^ at c and d respectively, and pass thence 
(the ray from c being transmitted) into the telescope at T. The direct 
vision grating-prism g may be swivelled in place or removed, at pleasure. 
To bring the system of four mirrors into complete parallelism is here 
of considerable importance if the spectrum fringes or the residual phe- 
nomenon are to be adequately large for measurement. The presence of 
the common mirror m, however suggests the procedure. When the 
clear plate P is in place, the rays ae and hf on returning are also again 
reflected at a and h toward Z, and may be clearly seen in a telescope* 
at p. Hence if m is the standard plane and nearly vertical, the mirrors 
Hi and Hi will be parallel when the slit images seen at p coincide hori- 
zontally and vertically, while Hi, Hi and m will have their common nor- 
mal plane in the diagram. In the same way the mirrors M and i^s may 
be parallelized with their common normal plane in the diagram. If the 
distances ac and hd, ah and cd have previously been made nearly equal 
and the angles approximately 90°, the fringes will usually be found on 
moving the micrometer screw normal to H3. 
As the mirrors are thick glass plates it is preferable that the half sil- 
vered sides of Hi and Hi be toward Z, and the half silvered side of Hz 
toward M. In this case each ray passes the plates twice, as indicated 
in figure 1. With ordinary plate glass the fringes when found are still 
apt to be small. They are then to be enlarged and centered, by com- 
pensator of clear glass C and C\ in the two rays respectively, rotated in 
opposite directions around a horizontal axis until the center of ellipses 
r 
FIG. 1. 
